Commitments and Contingencies - Letters of Credit and Guarantees (Detail)	Sep. 30, 2015 USD ($)
Tampa Electric Company [Member] | Letters Of Credit [Member]
Guarantee Obligations [Line Items]
Year of Expiration 2015	$ 0
Year of expiration 2016	0
Year of expiration 2017-2019	0
Year of expiration After 2019	500,000
Maximum Theoretical Obligation	500,000
Liabilities Recognized at June 30, 2015	100,000
Year of expiration 2016-2019	0
New Mexico Gas Company [Member] | Letters Of Credit [Member]
Guarantee Obligations [Line Items]
Year of Expiration 2015	0
Year of expiration 2016	0
Year of expiration 2017-2019	0
Year of expiration After 2019	1,700,000
Maximum Theoretical Obligation	1,700,000
Liabilities Recognized at June 30, 2015	0
Fuel Sales and Transportation [Member]
Guarantee Obligations [Line Items]
Year of Expiration 2015	0
Year of expiration 2016	0
Year of expiration 2017-2019	0
Year of expiration After 2019	92,900,000
Maximum Theoretical Obligation	92,900,000
Liabilities Recognized at June 30, 2015	0
Letters of Indemnity - Coal Mining Permits
Guarantee Obligations [Line Items]
Year of Expiration 2015	0
Year of expiration 2016	93,800,000
Year of expiration 2017-2019	0
Year of expiration After 2019	0
Maximum Theoretical Obligation	93,800,000
Liabilities Recognized at June 30, 2015	$ 0